Variable ride height vehicle suspension system

ABSTRACT

A variable ride height vehicle suspension system in which the energy input is from ground impact includes a telescopic hydropneumatic strut with a liquid chamber separated from a gas chamber by a free piston. A reservoir external of the strut is connected to one of the chambers via a valve arrangement also external of the chambers. The valve arrangement is selectively operable to permit unidirectional fluid flow in one or the other direction between the reservoir and the strut chamber to which it is connected, the flow direction being selected to enable lengthening or shortening of the strut in response to pressure oscillations in the gas chamber.

FIELD OF THE INVENTION

This invention relates to height adjustment mechanisms for vehicles. Ithas particular relevance to hydropneumatic suspensions.

BACKGROUND OF THE INVENTION

In hydropneumatic suspension systems a typical arrangement comprises apiston connected to a wheel or other ground engaging part such as acaterpillar track. The piston extends into a liquid chamber and forcesthe liquid from that chamber through a damping restrictor into a secondchamber. A floating piston or flexible membrane separates the secondchamber from a closed gas chamber so that liquid entering the secondchamber compresses the gas to provide the spring in the suspension. Inalternative arrangements a dashpot replaces the restricted passagebetween liquid chambers.

There are various limitations to such a system. One problem, which hasbeen addressed, is the change in height with loading due to compressionof the gas, and systems have been proposed to retain constant height byvarying the mass of gas in the chamber. However, existing proposalsrequire either a source of compressed gas and a venting means, the gaschamber then no longer being closed, such as described in U.S. Pat. No.4,408,773, or a two-part gas chamber with connecting valve and ahydraulic pumping arrangement to cause transfer of gas between the twochambers effectively to change the mass of gas in the part of thechamber forming the spring. This latter arrangement is shown in UKspecification 1602291.

These two specifications concern retaining constant height of chassis.On the other hand there are also instances when it is desired to adjustchassis height, irrespective of load, for example to enable vehicles totravel over different types of terrain, to vary the utilisation mode ofthe vehicle or to suit the speed of travel. Height adjustments such asraising the height of the chassis require significant energy input toraise the mass of the vehicle. Various `active` suspension systems havebeen designed in which this energy is provided through hydraulic,pneumatic or electrical servo mechanisms.

Such active suspension systems involving an external source of energydraw on the energy resources of the vehicle, thus leading to reducedfuel efficiency, extra weight and design complexity.

There are proposals for adjusting ride height that harness the veryconsiderable energy generated within the suspension system from groundsurface impacts and wheel patter. This energy, when not used, isnormally dispersed as heat, for example in the damping element ,of thesuspension system. However, the systems using this ground generatedenergy have been centred on the hydraulic part of hydropneumatic systemsand still suffer from design complexity.

UK specification 1128092 shows an arrangement in which valves in aported piston separating two hydraulic chambers are arranged to permitfluid flow from one chamber to another when the piston oscillates as aresult of ground impact. A staggered porting arrangement for the valvesprovides differential transfer between the chambers when the pistonoscillates about a position displaced from a predetermined mean, such asoccurs when load is increased. As a result of the differential transferthe piston and the chassis height are restored to the predeterminedmean. Such an arrangement provides automatic ride height adjustment,utilising ground impact energy, but does not provide selectable,variable ride height during vehicle motion.

SUMMARY OF THE INVENTION

The present invention is directed towards providing variable rideheight, which may also be utilised to retain constant height forvariable load (i.e. self-levelling), where the energy input is fromground impact and is simple in function, reliable and cheap tomanufacture.

Accordingly the invention provides a hydropneumatic suspension systemfor a vehicle comprising a chassis and ground engaging parts in whichenergy from ground impact of ground engaging parts is utilised to adjustthe height of the chassis relative to the ground engaging parts, thearrangement comprising a fluid chamber subject to bump and reboundpressures and having valved interconnections to a second chamberenabling flow of fluid between the chambers to provide the adjustment tothe height of the chassis characterised in that the second chamber is anauxiliary chamber not supporting the chassis and the valves are remotelyoperable selectively to raise or lower the chassis by permitting fluidflow respectively under bump or rebound pressures.

The invention also provides a telescopic hydropneumatic strut comprisinga liquid chamber separated from a gas chamber by a free piston, areservoir external of the strut connected to one of said chambers via avalve arrangement carried externally of the chambers, the valvearrangement being selectively operable to permit unidirectional fluidflow in one or other direction between the reservoir and the strutchamber to which it is connected, the flow direction being selected toenable lengthening or shortening of the strut resulting from pressureoscillations in the gas chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described by way of example with reference to theaccompanying drawings in which:

FIG. 1 is a schematic diagram of a strut assembly incorporating apreferred embodiment of the invention, showing the strut in the bumpposition;

FIG. 2 is the schematic diagram showing the strut of FIG. 1 in therebound position;

FIG. 3 shows gas spring characteristics for gas pressure versus struttravel; and

FIG. 4 is a schematic side elevation, in section, of a furtherembodiment of a strut assembly in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a hydropneumatic strut of a type suitable for attachment atone end 1 to a wheel (not shown) or other ground engaging mechanism. Theother end 2 of the strut is adapted for connection to a vehicle chassis(not shown) in such a manner that vehicle loads are transmitted via thestrut to the wheel or other ground engaging part.

The strut comprises an oil chamber 3 defined by cylindrical wall 4 andend 5 which can be seen more clearly in FIG. 2. A floating piston 6,which may alternatively be replaced by a flexible membrane, separatesthe oil chamber 3 from a gas chamber 7. The gas chamber is defined bycylindrical wall 8 and end 9, again more clearly shown in FIG. 2.

A damper 10 is carried by the end 11 of the cylindrical wall 8 remotefrom the end 9, the damper being located within the oil chamber and theend 11 of the cylindrical wall 8 being a sliding fit within thecylindrical wall 4 of the oil chamber 3. It will be appreciated that thearrangement may be varied, the strut essentially comprising a telescopicarrangement with relatively sliding parts and a floating piston orflexible member defining a hydraulic and pneumatic chamber.

It will be seen from FIG. 1 that when the strut is in the retractedconfiguration which occurs when the ground imparts a `bump` the relativemotion causes the damper 10 and end 11 of tube 8 to be pushed into theoil chamber. (The motion is perhaps more properly described as the oilchamber pushing on to the damper). The oil passes through the damper 10which is cylindrical or suitably apertured and into the narrowercylinder now defined by the end 11 of the gas cylinder 8, simultaneouslycompressing the gas by movement of the floating piston 6. Thisparticular damping arrangement may be modified, or damping providedwithin the gas chamber, without departing from the operational aspect ofthe invention which is now described.

A connection 12 links the main gas cylinder 7 via a valve assembly 13 toan auxiliary gas cylinder 14. The valve assembly 13 comprises two pilotor solenoid operated shut off valves 15 and 16, and two non-returnvalves 17 and 18. This valve arrangement is by way of example but otherswhich give the same effect may be used. Both the auxiliary gas cylinderand the valve assembly maybe chassis mounted on top of the strut ormounted near to it. In the latter case the connection 12 would be aflexible hose. For protection purposes they may beth be armoured.

A control box 20 provides operating signals to the shut-off valves 15and 16 and may itself be supplied with information from a manual controlunit 21 mounted in an operator or driver console or from amicroprocessor unit 22 which commutes commands based on inputinformation such as actual ride height, payload and/or predeterminedride height requirements. Each strut unit on a vehicle normally has itsown valve assembly and auxiliary gas cylinder in order to establish itsindependence from other strut units. This is particularly important formilitary vehicles so that battle damage does not cause a totalsuspension system failure. Non-military vehicles may have a centralisedauxiliary gas cylinder with either centralised or individual valveunits.

Considering the situation when the strut is set for low ride height, atthis stage the strut, relatively speaking, is telescoped together(closer to the mean height position which is half way between bump andrebound positions as shown in FIGS. 1 and 2) and the volume of the maingas cylinder 7 is at its static minimum. By static minimum is meant theminimum rest configuration, a lower or smaller minimum occurs on bump.The charge pressure of the gas in the auxiliary gas cylinder 14, in thisconfiguration, is selected to be approximately equal to the pressurethat is experienced in the main gas cylinder 7 when ground impact causesa rise in the wheels, transmitted via the oil cylinder as a compressionof cylinder 7. This pressure, as occurs in the position shown in FIG. 2,is known as the bump pressure.

When it is required to increase the ride height of the vehicle, valve 15is opened and valve 16 closed thus enabling flow from the auxiliary gascylinder 14 into the main cylinder 7. On changing to this valve settingthere is an initial flow of gas into the main gas cylinder because ofthe greater pressure in the auxiliary cylinder. This initial flow, untilthe pressures have equalised, causes an initial rise in the ride height.Non-return valve 18 prevents gas being forced back into the auxiliarycylinder in the event of a bump, thus essentially retaining the closednature of the main cylinder for gas spring purposes.

If no further increase in height is required valve 15 is closed. Shouldfurther height be required valve 15 is kept open. Then as the vehiclemanoeuvres and the wheels encounter roughness they rise and thenrebound. On the rebound the strut extends causing the gas in thecylinder 7 to expand further thus reducing its pressure below theoriginal value. The reduction in pressure in the main gas cylinder 7caused by the rebound brings its pressure below that of the auxiliarycylinder 14 and so further gas transfers into the main cylinder. Oncontraction after the rebound non return valve 18 prevents flow back tothe auxiliary cylinder 14. Incremental height increases are therebyachieved with each rebound either until the desired height is reachedand valve 15 closed or until maximum height is reached when the pressurein the auxiliary cylinder 14 has fallen to the same level as the reboundpressure of the main cylinder thereby preventing any further transfer.FIG. 2 shows the strut fully extended in the `rebound` position.

When it is desired to reduce the ride height, valve 15 is closed andvalve 16 is opened. If the lowering is from the maximum height then onopening the valve 15 the initial pressure in the auxiliary cylinder 14is at rebound pressure and lower than that of the main cylinder 7 sothat gas transfers from the main cylinder 7 to the auxiliary cylinder14, leading to an initial reduction in height, until the pressures haveequalised. Non return valve 17 prevents gas flowing in the reversedirection from auxiliary cylinder to main cylinder.

Now as the vehicle manoeuvres, wheel rise causes compression of the maingas cylinder 7, with the pressure rising to bump pressure, so that gascontinues to transfer with each bump into the auxiliary cylinder 14either until valve 7 is closed at the desired height or when theauxiliary cylinder reaches bump pressure at the minimum ride height. Onrebound after bumps and between bumps valve 17 prevents reverse flowback to the main gas cylinder 7.

The above description has been in terms of variable ride height. Morefundamentally the invention may be described as actuation of theeffective length or volume of a strut, utilising pressure oscillations.It will be realised that increasing the load leads to a reduction inride height and valve settings to increase height to a specific levelmay be used to maintain a given height under a variable load.

Monitoring pressure in the auxiliary chamber (and hence in the strut)may be used to provide a height reading for a given load. This could beachieved using a three way valve and a pressure transducer with acontroller unit inside the vehicle.

FIG. 3 illustrates the extreme characteristic curves for maximum andminimum loads of a typical strut design.

A similar mechanism may be used on the hydraulic side of ahydropneumatic system. In this instance as shown in FIG. 4 the hose lineextends from the oil cylinder 3 (which now constitutes the maincylinder) and the auxiliary cylinder 14 comprises an oil cylinder 14awith a floating piston 14b separating it from an enclosed gas chamber14c that provides volume and pressure adjustment. To lower the chassis,on bumping oil transfers to the auxiliary cylinder via a similar valvearrangement 13 as before and the gas part 14c of the auxiliary chambercompresses until bump pressure is reached. Conversely, to increaseheight on rebound, fluid transfers to the cylinder 3 and the gas part ofthe auxiliary cylinder expands until rebound pressure is reached.

We claim:
 1. A hydropneumatic system comprising:(I) a telescopic strutincluding(A) a first chamber including first and second mutuallytelescopically movable wall parts, said first chamber containinghydraulic fluid; (B) a second chamber containing pneumatic fluid; and(C) a free piston means dividing said first chamber from said secondchamber; (II) an auxiliary chamber containing pneumatic fluid; and (III)selectively operable valve means connecting said auxiliary chamber andsaid second chamber, said valve means comprising first valve means forallowing unidirectional flow of pneumatic fluid from said auxiliarychamber to said second chamber and second valve means for allowingunidirectional flow of pneumatic fluid from said second chamber to saidauxiliary chamber.
 2. A hydropneumatic system comprising:(I) atelescopic strut including(A) a first chamber including first and secondmutually telescopically movable wall parts, said first chambercontaining hydraulic fluid; (B) a second chamber containing pneumaticfluid; (C) a free piston means dividing said first chamber from saidsecond chamber; and (D) first and second ends separated by said firstand second chambers; (II) an auxiliary chamber containing pneumaticfluid; and (III) selectively operable valve means connecting saidauxiliary chamber and said second chamber, said valve means comprisingfirst valve means for allowing unidirectional flow of pneumatic fluidfrom said auxiliary chamber to said second chamber in response to ahigher pneumatic pressure in said auxiliary chamber than in said secondchamber and second valve means for allowing unidirectional flow ofpneumatic fluid from said second chamber to said auxiliary chamber inresponse to a higher pneumatic pressure in said second chamber than insaid auxiliary chamber, said unidirectional flow of pneumatic fluid tosaid second chamber causing increase in separation of said first andsecond ends, said unidirectional flow of pneumatic fluid to saidauxiliary chamber allowing decrease in the separation of said first andsecond ends.
 3. A hydropneumatic system comprising:(I) a telescopicstrut including(A) a first chamber including first and second mutuallytelescopically movable wall parts, said first chamber containinghydraulic fluid; (B) a second chamber containing pneumatic fluid; and(C) a free piston means dividing said first chamber from said secondchamber; (II) an auxiliary cylinder comprising a hydraulic chamber, agas chamber and a floating piston separating said hydraulic chamber fromsaid gas chamber; and (III) selectively operable valve means connectingsaid hydraulic chamber and said first chamber, said valve meanscomprising first valve means for allowing unidirectional flow ofhydraulic fluid from said hydraulic chamber to said first chamber inresponse to a higher pneumatic pressure in said gas chamber and secondvalve means for allowing unidirectional flow of hydraulic fluid fromsaid hydraulic chamber to said auxiliary chamber.
 4. A hydropneumaticsystem comprising:(I) a telescopic strut including(A) a first chamberincluding first and second mutually telescopically movable wall parts,said first chamber containing hydraulic fluid; (B) a second chambercontaining pneumatic fluid; (C) a free piston means dividing said firstchamber from said second chamber; and (D) first and second endsseparated by said first and second chambers; (II) an auxiliary cylindercomprising a hydraulic chamber, a gas chamber and a floating pistonseparating said hydraulic chamber from said gas chamber; and (III)selectively operable valve means connecting said hydraulic chamber andsaid first chamber, said valve means comprising first valve means forallowing unidirectional flow of hydraulic fluid from said hydraulicchamber to said first chamber in response to a higher pneumatic pressurein said gas chamber than in said second chamber, and second valve meansfor allowing unidirectional flow of hydraulic fluid from said hydraulicchamber to said auxiliary chamber in response to a higher pneumaticpressure in said first chamber than in said gas chamber, saidunidirectional flow of hydraulic fluid to said first chamber causingincrease in separation of said first and second ends, saidunidirectional flow of pneumatic fluid to said auxiliary chamberallowing a decrease in the separation of said first and second ends.